The present invention is directed to a method, and structure, which suppresses separation of fluid (e.g., gas or liquid, such as air) flow adjacent a surface of a body when passing the fluid along an exterior surface of the body, such as in the case of a fluid flowing across the wing of an airplane, or when passing the fluid along an interior surface of the body, such as in the case of a fluid flowing in a diffuser along a surface of the diffuser. In particular, the present invention is directed to a method, and structure, wherein instabilities or oscillations associated with separation of fluid flow adjacent the surface of the body is suppressed, and drag associated with this fluid flow separation is suppressed.
It is well known in fluid dynamics that when a fluid passes along a solid body, the fluid will form a viscous layer, known as a xe2x80x9cboundary layerxe2x80x9d, adjacent the body surface. This boundary layer possesses a much lower energy level than the flow outside it. Inside the boundary layer, the flow is distorted under viscous effects and there exists a large velocity gradient in the direction normal to the body surface.
When a large adverse pressure gradient exists, that is, when pressure increases in the direction of the fluid flow, the boundary layer may not be able to tolerate the pressure gradient, and will start to separate from the body surface, as schematically illustrated in FIG. 1. Thus, shown in FIG. 1 is surface 1 of the body, having fluid flow adjacent surface 1. The flow is shown schematically by reference character 3, and includes, initially, flow adjacent surface 1 of the body (attached flow, e.g., in an attached boundary layer flow region), shown by reference character 5, and separated flow, e.g., in separated boundary layer flow region shown by reference character 7, where the flow is separated from surface 1 of the body. The separation point, where the flow initially separates from the body, is shown by reference character 9 in FIG. 1. This point is defined as the point where the velocity gradient in the direction normal to the body surface is observed for the first time to be continuously less than or equal to zero.
FIGS. 18(a) and 18(b) respectively show, in more detail (with respect to fluid flow) than shown in FIG. 1, such separation from an external surface 1 of a body 2 (e.g., an airplane wing) and from an internal surface 1a of a body 2a (e.g., a diffuser). This phenomenon is called xe2x80x9cflow separationxe2x80x9d, or xe2x80x9cseparationxe2x80x9d. Shown in FIG. 18(a) is separation point 9, where the fluid first separates from surface 1 of body (airfoil) 2. Downstream from the separation point 9, in the direction of fluid flow, the fluid exhibits separated flow in a separated flow region 13 (separated boundary layer flow region), where the fluid flow forms eddies 17. Even downstream of separation point 9, in regions spaced from surface 1 or 1a of the body of the fluid exhibits smooth flow, e.g., smooth outer flow, in smooth outer flow regions 16 shown in each of FIGS. 18(a) and 18(b). The separated boundary layer flow region is defined as the region in which the local flow velocity is directed essentially in the direction opposite to the direction of the main flow prior to the separation point. This is compared to the smooth flow region (smooth outer flow region), which is the flow region outside (relative to the body) the separated boundary layer flow region.
Moreover, as the fluid flow continues over time this flow separation propagates upstream from an initial separation point, as seen in FIGS. 2(a) and 2(b), and as shown in more detail with respect to fluid flow patterns in FIGS. 19(a) and 19(b). That is, FIG. 2(a) shows an initial stage of fluid flow, and FIG. 2(b) shows a later stage (later in time). At the initial stage, separation point 9 is toward the rear end of surface 1 of the body, with respect to fluid flow direction 3. At a later time, separation point 15 has moved upstream, as shown in FIG. 2(b). And, as shown in FIG. 19(a) disturbance waves 11 propagate upstream from inside the separated flow region 13, exerting influences on the upstream flow region. Separated flow region 13 is extended upstream, so that separation point 9 changes to an xe2x80x9cadjustedxe2x80x9d separation point 15, as shown in FIG. 19(b).
In almost all cases, flow separation is associated with disadvantages, and is therefore to be avoided. If the body is the wing of an airplane, flow separation may cause the airplane to lose its lifting force, a situation known as xe2x80x9cstallxe2x80x9d. Flow separation also increases the drag force acting on the wing, which is particularly disadvantageous when the airplane is in a cruise condition. If separation occurs inside a diffuser, the diffuser loses its diffusing ability.
In many cases, flow separation also leads to the occurrence of unsteady phenomena, which may cause control problems. Aerodynamic unsteadiness could alternately lead to structural failure of the body in question. This is particularly true for an airplane flying in the flow regime known as the xe2x80x9ctransonicxe2x80x9d regime, or maneuvering a landing approach, where a phenomenon known as xe2x80x9cbuffetingxe2x80x9d can arise due to flow separations. Flow separation also often leads to other interfering phenomena, such as the disturbing wind noises around transport vehicles, or the noise interference to electrical/power transmissions through cables/power lines.
In the transonic regime, where the flow speed is close to the speed of sound, separation induced flow oscillations (usually in conjunction with the formation of shock-waves) can be very severe. One method that has been proposed to stop these oscillations is disclosed in U.S. Pat. No. 5,692,709 to Mihora and Cannon. In this particular patent a method is described in which flow oscillations are stopped by fixing simple devices at prescribed positions on the surface of the aerodynamic body. These devices force shock-waves to form prematurely at fixed locations, which are the locations of these devices.
In addition to the foregoing, many other methods have been proposed to suppress flow separation under specific flow conditions. These include, but are not limited to, vortex generators (such as disclosed in U.S. Pat. No. 5,253,828 to Cox), riblets (such as disclosed in U.S. Pat. No. 4,863,121 to Savill), large-eddy break-up devices, porous or slotted walls, fluid blowing and/or suction, moving surfaces, actuators (such as disclosed in U.S. Pat. No. 5,209,438 to Wygnanski), vibrating flexible structure (such as U.S. Pat. No. 5,961,080 to Sinha), and stepped body surfaces.
As mentioned previously, flow separation is associated with the existence of vortical structures (eddies) 17, of various sizes, inside separated flow region 13, as shown in FIGS. 18(a) and 18(b). These eddies 17 give rise to disturbance waves 11 (see FIG. 19(a)), which can travel in the upstream direction. The disturbance information in the disturbance waves 11 is received by the flow upstream of the original separation point. The flow will adjust itself to this information, and the separation point is shifted upstream, until some kind of balance (e.g., steady-state) is achieved. This adjustment of original separation point 9 to adjusted separation point 15 (see FIG. 19(b)), at an upstream location from original separation point 9 due to this disturbance information, has been previously mentioned. In the case that the disturbances are large, the whole flow field is said to be xe2x80x9cunsteadyxe2x80x9d, and the point of separation fluctuates about a mean position. The flow field then constitutes a feed-back system.
As mentioned previously, on the surface of the body the fluid particles form a very thin viscous layer, known as the xe2x80x9cboundary layerxe2x80x9d. Because flow inside the boundary layer is mostly much slower than the flow outside it, it is easier for disturbances to travel inside this layer. In the case that the flow outside the boundary layer is supersonic, upstream propagation can occur only inside the boundary layer. Therefore, preventing the disturbances from propagating upstream inside the boundary layer is important in controlling flow separation, especially in high-speed flows.
The present invention is intended to lessen this upstream propagation or upstream influences of disturbances that originate inside the separated flow region. It is meant not only to reduce drag associated with the flow separation, but also suppress the instabilities or oscillations associated with the flow separation. The method and structure of the present invention involve, as an illustrative embodiment, providing a barrier member (for example, a tab), or a plurality of barrier members (e.g., a plurality of tabs), on the body surface, each barrier member acting as a physical barrier extending away from the surface of the body, and which prevents the upstream propagation of the disturbances, as illustrated schematically in FIG. 3, and prevents upstream influences of the disturbances.
That is, FIG. 3 shows schematically barrier member (tab) 19 according to the present invention. Barrier member 19 blocks upstream propagation of disturbances, for example, those disturbances excited by eddies 17.
As can be seen from FIG. 3, barrier member 19 in FIG. 3 extends from surface 1 of the body into separated flow region, but does not extend beyond edge 21, the boundary of separated flow region 13, into smooth flow region 16. That is, in the absence of barrier member 19, there is a separated flow region 13 adjacent surface 1 of the body and smooth flow region 16 outside separated flow region 13 (with respect to surface 1). According to the present invention, the at least one barrier member extends into this separated flow region absent the barrier member, but not into the smooth flow region.
Due to barrier member 19 blocking upstream propagation of disturbances originating in separated flow region 13, separation point 9 is maintained substantially without moving upstream. This barrier member 19 penetrates into the separated flow region 13, but not into the outer, smooth flow region 16. The length and angle of deflection of barrier member or members 19, with respect to surface 1 of the body, can be varied according to the severity of the separation.
Thus, the present invention involves converting part of the surface affected by the fluid flow separation, from an originally smooth surface into one having barrier members, e.g., barrier members such as steps, tabs, etc. These steps, tabs, etc., act as barriers that prevent upstream propagation, or upstream influences, of disturbances. If the steps, tabs, etc. are kept within the separated region, they would not interfere with the outer, smooth flow.